analysis of the responses across a population of
receptors to decipher their various wavelengths
and intensities. This ‘triangulation’ enables us to
spot the poisonous berries among the nutritious
leaves. Neurons called opponent cells simplify
the algebra in much the same way that the
RGB chromaticity coordinates in the CIE colour
space are ‘normalised’ (made to add up to one)
mathematically so that only two values are needed
to describe the properties of a light source.

The human system works miraculously wellon the whole but can be fooled; some sourceswith different spectral power distributionswill appear the same – when, for example, redlight with a particular intensity exacts the sameresponse as green light of a different but specificintensity. This phenomenon, which occurs moreoften with near-neutral or dark colours, is calledmetamerism and is akin to jamming a radiosignal. Sources can be metameric. Two samplesof materials that appear to be the same colourunder one source may not under another.

The information sent along the optic nerveis processed in a region of the brain knownas visual area 4 or V4. In addition to colour,brightness and texture, the brain processes shape,orientation, curvature, motion and depth, andin doing so extrapolates everything we ‘see’ fromtiny amounts of fragmentary information. Thevisual system also filters out a lot of extraneousinformation. Our eyes constantly judder andare full of floating gunk that gets in the way ofthe light passing through to the retina. The lensitself fluoresces, especially when exposed to‘When “coloured light” reachesthe eye it is transformed into aform of abstract data’